Problem: 6 people can paint 5 walls in 43 minutes. How many minutes will it take for 8 people to paint 9 walls? Round to the nearest minute.
Explanation: We know the following about the number of walls $w$ painted by $p$ people in $t$ minutes at a constant rate $r$ $w = r \cdot t \cdot p$ $\begin{align*}w &= 5\text{ walls}\\ p &= 6\text{ people}\\ t &= 43\text{ minutes}\end{align*}$ Substituting known values and solving for $r$ $r = \dfrac{w}{t \cdot p}= \dfrac{5}{43 \cdot 6} = \dfrac{5}{258}\text{ walls painted per minute per person}$ We can now calculate the amount of time to paint 9 walls with 8 people. $t = \dfrac{w}{r \cdot p} = \dfrac{9}{\dfrac{5}{258} \cdot 8} = \dfrac{9}{\dfrac{20}{129}} = \dfrac{1161}{20}\text{ minutes}$ $= 58 \dfrac{1}{20}\text{ minutes}$ Round to the nearest minute: $t = 58\text{ minutes}$